“Green” chemistry

In mechanochemistry, reagents are finely ground and mixed so that they combine to form the desired product, even without need for solvent. By eliminating solvent, this technology promises to contribute significantly towards ‘green’ and environmentally benign chemical manufacture in the future. However, there are still major gaps in understanding the key processes that occur during mechanical treatment and reaction. A team led by the Federal Institute for Materials Research (BAM) has now developed a method at BESSY II to observe these processes in situ with X-ray scattering.

Chemical reactions are often based on the use of solvents that pollute the environment. Yet, many reactions can also work without solvent. This is the approach known as mechanochemistry, in which reagents are very finely ground and mixed together so that they react with each other to form the desired product.  The mechanochemical approach is not only more environmentally friendly, but even potentially cheaper than classical synthesis methods. The International Union of Pure and Applied Chemistry (IUPAC) therefore ranks mechanochemistry among the 10 chemical innovations that will change our world. However, the full potential of this technology cannot be realized until the processes during mechanical treatment are understood in more detail, so that it is possible to precisely direct and control them.

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Image: Finely ground powders can also react with each other without solvents to form the desired product. This is the approach of mechanochemistry.

Credit: © F. Emmerling/BAM

Spintronics: Exotic ferromagnetic order in two-dimensions

An international team has detected at HZB’s vector magnet facility VEKMAG an unusual ferromagnetic property in a two-dimensional system, known as “easy-plane anisotropy”. This could foster new energy efficient information technologies based on spintronics for data storage, among other things. The team has published its results in the renowned journal Science.

The thinnest materials in the world are only a single atom thick. These kinds of two-dimensional or 2D materials – such as graphene, well-known as consisting of a single layer of carbon atoms – are causing a great deal of excitement among research teams worldwide. This is because these materials promise unusual properties that cannot be obtained using three-dimensional materials. As a result, 2D materials are opening the door to new applications in fields such as information and display technology, as well as for critical components in extremely sensitive sensors.

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Image: STM topography of a monolayer CrCl3 grown on Graphene/6H-SiC(0001). Inset, a magnified topography image, which reveals the grain boundaries.

Credit: HZB

Ultrafast magnetism: heating magnets, freezing time

Magnetic solids can be demagnetized quickly with a short laser pulse, and there are already so-called HAMR (Heat Assisted Magnetic Recording) memories on the market that function according to this principle. However, the microscopic mechanisms of ultrafast demagnetization remain unclear. Now, a team at HZB has developed a new method at BESSY II to quantify one of these mechanisms and applied it to the rare-earth element Gadolinium, whose magnetic properties are caused by electrons on both the 4f and the 5d shells. This study is completing a series of experiments done by the team on Nickel, Iron-Nickel Alloys. Understanding these mechanisms is useful for developing ultrafast data storage devices.

New materials should make information processing more efficient, for example, through ultrafast spintronic devices that store data with less energy input. But to date, the microscopic mechanisms of ultrafast demagnetization are not fully understood. Typically, the process of demagnetization is studied by sending an ultrashort laser pulse to the sample, thereby heating it up, and then analyzing how the system evolves in the first picoseconds afterward.

Read more on the HZB website.

Image: The picture shows the glowing filament which keeps the sample at constant temperatures during the measurements.

Credit: © HZB

Beam diagnostics for future laser wakefield accelerators

For decades, particle accelerators have been getting bigger and bigger. In the meantime, ring accelerators with circumferences of many kilometres have reached a practical limit. Linear accelerators in the GHz range also require very long construction lengths. For some years now, however, an alternative is explored: “tabletop particle accelerators” based on the laser excitation of charge waves in plasmas (laser wakefield). Such compact particle accelerators would be particularly interesting for future accelerator-driven light sources, but are also being investigated for high-energy physics. A team from Helmholtz-Zentrum Berlin (HZB) and the Physikalisch-Technische Bundesanstalt (PTB) has developed a method to precisely measure the cross-section of electron bunches accelerated in this way.  This brings applications of these new accelerator technologies for medicine and research closer.

The principle of laser wakefield accelerators: A high-power laser excites a charge wave in a plasma, which propagates at the speed of the laser pulse and pulls electrons behind it in its “wake”, thus accelerating them. Electron energies in the GeV range have been achievable with this technique for some time. However, the electron bunches produced in this way have so far been too small and too poorly focused to use the synchrotron radiation they emit, an intense, coherent light that is used for research in many different disciplines.

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Image: Information on beam quality can be extracted via the interference patterns at different focal lengths and photon intensities.

Credit: © http://www.nature.com/articles/s42005-021-00717-x

X-ray microscopy with 1000 tomograms per second

Tomoscopy is an imaging method in which three-dimensional images of the inside of materials are reconstructed in rapid succession. A new world record has now been set at the Swiss Light Source at the Paul Scherrer Institute: with 1000 tomograms per second, it is now possible to non-destructively capture very fast processes and structural changes in materials on the micrometre scale, such as the burning of a sparkler or the foaming of a metal alloy for the production of stable lightweight materials. 

Most people are familiar with computed tomography from medicine: a part of the body is X-rayed from all sides and a three-dimensional image is then calculated, from which any sectional images can be created for diagnosis.

This method is also very useful for material analysis, non-destructive quality testing or in the development of new functional materials. However, to examine such materials with high spatial resolution and in the shortest possible time, the particularly intense X-ray light of a synchrotron light source is required. In the synchrotron light, even rapid changes and processes in material samples can be visualised if it is possible to capture 3-dimensional images in a very short time sequence.

A team led by Francisco García Moreno from the Helmholtz Centre Berlin is working on this, together with researchers from the Swiss Light Source SLS at the Paul Scherrer Institute (PSI). Two years ago, they managed a record 200 tomograms per second, calling the method of fast imaging “tomoscopy”. Now the team has achieved a new world record: with 1000 tomograms per second, they can now record even faster processes in materials or during the manufacturing process. This is achieved without any major compromises in the other parameters: the spatial resolution is still very good at several micrometres, the field of view is several square millimetres and continuous recording periods of up to several minutes are possible.

Read more on the PSI website

Also on the HZB website

Image: Christian Schlepütz at the Tomcat beamline of the Swiss Light Source SLS, where a team of scientists have developed a 3D imaging method capable of recording 1,000 tomograms per second.

Credit: Paul Scherrer Institute/Mahir Dzambegovic

Sharper insights into thin-film systems

Interfaces in semiconductor components or solar cells play a crucial role for functionality. Nevertheless, until now it has often been difficult to investigate adjacent thin films separately using spectroscopic methods. An HZB team at BESSY II has combined two different spectroscopic methods and used a model system to demonstrate how well they can be distinguished.

Photoelectron spectroscopy (PES) enables the chemical analysis of surfaces and semiconductor layers. In this process, an X-ray pulse (photons) hits the sample and excites electrons to leave the sample. With special detectors, it is then possible to measure the direction and binding energy of these electrons and thus obtain information about electronic structures and the chemical environment of the atoms in the material. However, if the binding energies are close to each other in adjacent layers, then it is hardly possible to distinguish these layers from each other with PES.

 A team at HZB has now shown how precise assignments can nevertheless be achieved: they combined photoelectron spectroscopy with a second spectroscopic method: Auger electron spectroscopy. Here, photoelectrons and Auger electrons are measured simultaneously, which gives the resulting method its name: APECS for Auger electron photoelectron coincidence spectroscopy (APECS). 

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Image: The illustration shows how the APECS measurement works on a nickel single crystal with an oxidised surface. An X-ray beam ionises atoms, either in the nickel crystal or on the surface. The excited photoelectrons from the surface and from the crystal have slightly different binding energies. The Auger electrons make it possible to determine the origin of the photoelectrons. 

Credit: © Martin Künsting /HZB

Disorder brings out quantum physical talents

Quantum effects are most noticeable at extremely low temperatures, which limits their usefulness for technical applications. Thin films of MnSb2Te4, however, show new talents due to a small excess of manganese. Apparently, the resulting disorder provides spectacular properties: The material proves to be a topological insulator and is ferromagnetic up to comparatively high temperatures of 50 Kelvin, measurements at BESSY II show.  This makes this class of material suitable for quantum bits, but also for spintronics in general or applications in high-precision metrology.

Quantum effects such as the anomalous quantum Hall effect enable sensors of highest sensitivity, are the basis for spintronic components in future information technologies and also for qubits in quantum computers of the future. However, as a rule, the quantum effects relevant for this only show up clearly enough to make use of them at very low temperatures near absolute zero and in special material systems.

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Image: The Dirac cone is typical for topological insulators and is practically unchanged on all 6 images (ARPES measurements at BESSY II). The blue arrow additionally shows the valence electrons in the volume. The synchrotron light probes both and can thus distinguish the Dirac cone at the surface (electrically conducting) from the three-dimensional volume (insulating).

Credit: © HZB

Review of X-ray scattering methods with synchrotron radiation

Synchrotron light sources provide brilliant light with a focus on the X-ray region and have enormously expanded the possibilities for characterising materials. In the Reviews of Modern Physics, an international team now gives an overview of elastic and inelastic X-ray scattering processes, explains the theoretical background and sheds light on what insights these methods provide in physics, chemistry as well as bio- and energy related themes.

“X-ray scattering can be used to investigate and resolve a wide variety of issues from the properties and excitations of fuctional solids, to homogeneous and heterogeneous chemical processes and reactions or even the proton pathway during the splitting of water,” explains Prof. Dr. Alexander Föhlisch, who heads the Institute Methods and Instrumentation for Research with Synchrotron Radiation at HZB.

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Image: Resonant X-ray excitation (purple) core excites the oxygen atom within a H2O molecule. This causes ultrafast proton dynamics. The electronic ground state potential surface (bottom) and the bond dynamics is captured by distinct spectral features in resonant inelastic X-ray scattering (right).

Credit: © Martin Künsting /HZB

Green hydrogen: Why do certain catalysts improve in operation?

Crystalline cobalt arsenide is a catalyst that generates oxygen during electrolytic water splitting in the production of hydrogen. The material is considered to be a model system for an important group of catalysts whose performance increases under certain conditions in the course of electrolysis. Now a HZB-team headed by Marcel Risch has observed at BESSY II how two simultaneous mechanisms are responsible for this. The catalytic activity of the individual catalysis centres decreases in the course of electrolysis, but at the same time the morphology of the catalyst layer also changes. Under favourable conditions, considerably more catalysis centres come into contact with the electrolyte as a result, so that the overall performance of the catalyst increases.

As a rule, most catalyst materials deteriorate during repeated catalytic cycles – they age. But there are also compounds that increase their performance over the course of catalysis. One example is the mineral erythrite, a mineral compound comprising cobalt and arsenic oxides with a molecular formula of (Co3(AsO4)2∙8H2O). The mineral stands out because of its purple colour. Erythrite lends itself to accelerating oxygen generation at the anode during electrolytic splitting of water into hydrogen and oxygen.

Read more in the HZB website

Image: Schematic of the electrochemical restructuring of erythrite. The fine needle-like structure melts during the conversion from a crystalline material to an amorphous one, which is porous like a Swiss cheese.

Credit: © HZB

When vibrations increase on cooling: Anti-freezing observed

An international team has observed an amazing phenomenon in a nickel oxide material during cooling: Instead of freezing, certain fluctuations actually increase as the temperature drops. Nickel oxide is a model system that is structurally similar to high-temperature superconductors. The experiment, which was carried out at the Advanced Light Source (ALS) in California, shows once again that the behaviour of this class of materials still holds surprises.

In virtually all matter, lower temperatures mean less movement of its microscopic components. The less heat energy is available, the less often atoms change their location or magnetic moments their direction: they freeze. An international team led by scientists from HZB and DESY has now observed for the first time the opposite behaviour in a nickel oxide material closely related to high-temperature superconductors. Fluctuations in this nickelate do not freeze on cooling, but become faster.

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Image: The development of this speckle pattern over time reveals microsocopic fluctuations in the material.

Credit: © 10.1103/PhysRevLett.127.057001

Water as a metal

Under normal conditions, pure water is an almost perfect insulator. Water only develops metallic properties under extreme pressure, such as exists deep inside of large planets. Now, an international collaboration has used a completely different approach to produce metallic water and documented the phase transition at BESSY II. The study is published now in Nature.

Every child knows that water conducts electricity – but this refers to “normal” everyday water that contains salts. Pure, distilled water, on the other hand, is an almost perfect insulator. It consists of H2O molecules that are loosely linked to one another via hydrogen bonds. The valence electrons remain bound and are not mobile. To create a conduction band with freely moving electrons, water would have to be pressurised to such an extent that the orbitals of the outer electrons overlap. However, a calculation shows that this pressure is only present in the core of large planets such as Jupiter.

Providing electrons

An international collaboration of 15 scientists from eleven research institutions has now used a completely different approach to produce a aqueous solution with metallic properties for the first time and documented this phase transition at BESSY II. To do this, they experimented with alkali metals, which release their outer electron very easily.

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Image: The picture on the top left shows an NaK drop in a vacuum without water vapour. The other pictures show the development of this drop over time when water vapour is present. Thus, a gold-coloured layer of metallic water forms first, followed by white spots of alkali hydroxide. After about 10 seconds, the drop falls.

Credit: © HZB/Nature

Universal mechanism of regulation in plant cells discovered

In pioneering work, a German-Japanese research team at BESSY II has been able to determine the 3D structure of a metalloprotein that plays an important role as a catalyst in all plant cells. This involves the DYW deaminase domain of what is referred to as the RNA editosome. The DYW domain alters messenger RNA nucleotides in chloroplasts and mitochondria and contains a zinc ion whose activity is controlled by a very unusual mechanism. The team has now been able to describe this mechanism in detail for the first time. Their study, published in Nature Catalysis, is considered a breakthrough in the field of plant molecular biology and has far-reaching implications for bioengineering.

All plant cells obtain their energy mainly from two organelles they contain – chloroplasts (responsible for photosynthesis) and mitochondria (responsible for the biochemical cycle of respiration that converts sugars into energy). However, a large number of a plant cell’s genes in its mitochondria and chloroplasts can develop defects, jeopardising their function. Nevertheless, plant cells evolved an amazing tool called the RNA editosome (a large protein complex) to repair these kinds of errors. It can modify defective messenger RNA that result from  defective DNA by transforming (deamination) of certain mRNA nucleotides.

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Image: Around the catalytic centre is a group of molecules, the gating domain, which can occupy two different positions.

Credit: © M. Künsting / HZB

Perovskite Solar Cells: Insights into early stages of structure formation

Using small-angle scattering at the PTB X-ray beamline of BESSY II, an HZB team was able to experimentally investigate the colloidal chemistry of perovskite precursor solutions used for solar cell production. The results contribute to the targeted and systematic optimization of the manufacturing process and quality of these exciting semiconductor materials.

Halide perovskite semiconductors are inexpensive, versatile, and high-performance materials used in solar cells as well as optoelectronic devices. The crystalline perovskite thin films required for this purpose are prepared at low temperature from solution: While the solvent evaporates during an annealing step, highly coordinated iodoplumbates interact and subsequently form the polycrystalline thin film. The quality of the thin film ultimately determines the performance of the semiconductor material. Up to now, it has not been possible to achieve a comprehensive impression of the role of the colloidal chemistry in the precursor that is considered to be directional for crystallinity and the further processing.

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Image: Using Small-Angle Scattering the early stages of structure formation in precursor solutions of perovskite solar cells have been explored.

Credit: Image: © M. Flatken/HZB

New insights into switchable MOF structures at the MX beamlines

Metal-organic framework compounds (MOFs) are widely used in gas storage, material separation, sensor technology or catalysis. A team led by Prof. Dr. Stefan Kaskel, TU Dresden, has now investigated a special class of these MOFs at the MX beamlines of BESSY II. These are “switchable” MOFs that can react to external stimuli. Their analysis shows how the behaviour of the material is related to transitions between ordered and disordered phases. The results have now been published in Nature Chemistry.

Metal-organic framework compounds (MOFs) consist of inorganic and organic groups and are characterised by a large number of pores into which other molecules can be incorporated. MOFs are therefore interesting for many applications, for example for the storage of gases, but also for substance separation, sensor technology or catalysis. Some of these MOF structures react to different guest molecules by changing their structures. They are thus considered switchable.

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Image: View into a MOF crystal exemplified by DUT-8. The massive pores are clearly discernible.

Credit: © TU Dresden

Direct observation of the ad- and desorption of guest atoms into a mesoporous host

Battery electrodes, storage devices for gases, and some catalyst materials have tiny functional pores that can accommodate atoms, ions, and molecules. How these guest atoms are absorbed into or released from the pores is crucial to understanding the porous materials’ functionality. However, usually these processes can only be observed indirectly. A team from the Helmholtz Zentrum Berlin (HZB) has employed two experimental approaches using the ASAXS instrument at the PTB X-ray beamline of the HZB BESSY II synchrotron to directly observe the adsorption process of atoms in a mesoporous model system. The work lays the foundations for new insights into these kinds of energy materials.

Most battery materials, novel catalysts, and storage materials for hydrogen have one thing in common: they have a structure comprised of tiny pores in the nanometer range. These pores provide space which can be occupied by guest atoms, ions, and molecules. As a consequence, the properties of the guest and the host can change dramatically. Understanding the processes inside the pores is crucial to develop innovative energy technologies.

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Image: From the measurement data, the team was able to determine that the xenon atoms first accumulate on the inner walls of the pores (state 1), before they fill them up (state 2). The X-ray beam penetrates the sample from below.

Credit: © M. Künsting/HZB

Towards catalysts for solar hydrogen production

Thin films of molybdenum and sulfur belong to a class of materials that can be considered for use as photocatalysts. Inexpensive catalysts such as these are needed to produce hydrogen as a fuel using solar energy. However, they are still not very efficient as catalysts. A new instrument at the Helmholtz-Berlin Zentrum’s BESSY II now shows how a light pulse alters the surface properties of the thin film and activates the material as a catalyst.

MoS2 thin films of superposed alternating layers of molybdenum and sulfur atoms form a two-dimensional semiconducting surface. However, even a surprisingly low-intensity blue light pulse is enough to alter the properties of the surface and make it metallic. This has now been demonstrated by a team at BESSY II.

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Image: A new instrument at BESSY II can be used to study molybdenum-sulfide thin films that are of interest as catalysts for solar hydrogen production. A light pulse triggers a phase transition from the semiconducting to the metallic phase and thus enhances the catalytic activity.

Credit: © Martin Künsting /HZB